[tax]

Adding for reserch and reference for later on [taken from airsoftplayers.com]

P90 cylinder's useable volume is actually about 11cc. Cylinder specs: diameter 23.84mm; useable length 24.81mm (31.04mm - 6.23mm cylinder head thickness). So with this information, the optimal barrel volume should be about 7.3cc (2/3 of 11cc). Using the stock bore (6.08mm), the optimal barrel length should be 251mm. So P90's stock barrel (247mm) is already optimal.

If you want to use a tightbore (6.04mm) 455mm long barrel (~13cc), the optimal cylinder volume should be about 19.5cc (13cc * 1.5) or 26cc (13cc * 2). By taping the cylinder air bleed hole, you end up with a type-0 cylinder (~27cc) with slightly less useable volume due to the bleed hole gap. It should work provided that air loss is not too great.

Another method you can try to bring out more performance (muzzle velocity) is changing the weight of the piston/piston head assembly. Short barrel AEGs perform better with lighter weight piston assembly; whereas long barrel AEGs perform better with heavier weight piston assembly.